The majority of aerosol particles in indoor occupational environments are 10-500 nm in diameter. Recently, Cohen et al. (1995) observed in a study with human lung casts that the charged particles deposit five times more efficiently than the zero-charge particles in the size range 20-200 nm. The significant implications of this increase in deposition include: (1) the enhanced deposition that results from particle charge will be important in airway dosimetry in the size range 10-500 nm since the deposition by diffusion and electrostatic charge effects are of the same order of magnitude for particles in this size range; (2) the increase in deposition from charge will need to be incorporated into general models used to assess occupational lung exposure. However, little is currently known about how the charge is distributed with particle size in indoor occupational environments. The goal of the proposal is to obtain data on charge distributions of aerosols representative of indoor occupational environments. The first step is to develop a system based on the principles of the Tandem Differential Mobility Analyzer to measure the charge distributions of particles in the range of 10-500 nm. This system will be capable of measuring the fractions of zero-, singly- and doubly-charged particles. The performance of this system will be evaluated with particles in equilibrium charge distribution. The second step is to employ this system to characterize the charge distributions of aerosols in indoor occupational environments. They include cigarette smoke aerosol, aerosols from a kerosene heater, a vacuum cleaner, an ultrasonic humidifier and cooking smoke. The obtained data will be incorporated into a current lung model to investigate the significance of electrostatic charge on lung airway dosimetry in indoor occupational environments. The proposed research will provide the experimental basis for a better model to estimate the importance of electrostatic charge effects on particle deposition in human airways leading to a more accurate risk assessment of occupational exposure of inhaled toxicant particles.